Silicon wafers are important elements of the semiconductor industry. Such wafers typically are sawn from artificially-grown silicon crystals and require one or more surface finishing or conditioning operations, each of which causes progressive thinning of a wafer.
The semiconductor industry is moving toward thinner wafers, especially important for smart cards, flash memory, and other products requiring thin packaging. One of the challenges for thin wafer manufacturing is uniformity of thickness after backgrind. A system is needed for accurate measurement of wafer thickness, preferably a non-contact system. Such a system may also be useful for measuring thickness of other semiconductor elements such as bare silicon wafers, flat panel displays, and MEMS.
A wafer starts at a thickness of approximately 0.8 mm and provides the building block for all of the processing leading to a wide range of electronic memory products. A finished wafer moves to a “backend” facility, which may be another plant in a different country. In the so-called backend segment of the semiconductor industry, the wafer goes through a back-grinding process wherein material is removed from the wafer backside by grinding the back surface. Currently, the wafer thickness after backgrinding is typically between about 0.1 mm and about 0.3 mm, depending upon the final application.
The factory segment of the industry consists of thinning, dicing, and packaging chips cut from the wafer.
There are at least two prior art methods and apparatus for non-contact measurement of thickness after backgrind of a finished wafer.
Systems employing capacitative sensors, for example, are available from ADE Technologies, Inc. (Westwood, Mass.) and Mechanical Technologies, Inc. (Albany, N.Y.). This technology has been used in measuring thickness of bare and patterned wafers. A known problem in applying this technology to wafers having chips formed on their surfaces is that the capacitative sensor can be confused by the signal from the chip and the wafer due to metal and/or other materials in the chip. Also, the capacitative technique can be influenced by a protective tape used to protect the wafer front surface.
Systems employing an optical type measurement are available from Frontier Semiconductor Measurements, Inc. (San Jose, Calif. Their system relies on measuring the interference of two beams from the top and bottom (after transmission through the part) surfaces. A known problem with this technology is that the spatial resolution can be relatively low, leading to confusion and overlap between a chip and another nearby feature on the wafer. Another problem is that the thickness measurement is dependent on knowing the index of refraction of the material.
What is needed in the art is a non-contact means for measuring the thickness of a test object, and especially of a chip-bearing silicon wafer after backgrind, wherein the thickness measurement is highly accurate, wherein spatial resolution along the surface of the object is very high, and wherein the means is not affected by metallic or non-silicon components of a wafer.
It is a principal object of the present invention to provide high-resolution, high-accuracy thickness measurement of a test object.